Re: [TLS] Last Call: <draft-ietf-tls-downgrade-scsv-03.txt> (TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks) to Proposed Standard

[Brian Smith <[e-mail]>]

Adam Langley <[e-mail]> wrote:
> On Fri, Jan 16, 2015 at 12:03 PM, Hanno Böck <[e-mail]> wrote:
>> Recently Mozilla has disabled the now so-called protocol dance, which
>> makes adding another workaround (SCSV) pretty much obsolete:
>
> Until they add TLS 1.3 support, when they'll need it again.

I don't think so, because we can change the way versions are
negotiated for TLS 1.3, so that the issue doesn't arise. In
particular, we can keep ClientHello.client_version as 0x0303 (TLS 1.2)
and negotiate TLS 1.3 with an extension.

Also, the rate of TLS 1.3 intolerance might be significantly lower
than projected. Ivan's numbers are based on a ClientHello with 0x0304
(TLS 1.3) as the record-layer version number. We know from past
experience working on NSS that 0x0301 (TLS 1.0) is a more compatible
record-layer version number. I think it was established that many
servers work fine when ClientHello.client_version = 0x0304 (TLS 1.3)
as long as the record-layer version number is 0x0301 (TLS 1.0) but
break when then record-layer vsion is 0x0304 (TLS 1.3). We'll need to
measure this in a more definitive way, but there's reason to be
optimistic.

Cheers,
Brian